Semiconductor wafer processing for microchip fabrication requires, among other steps, sequential and repeated steps such as masking, deposition, and etching. In the etching step, the wafer and the chamber in which the etching takes place are exposed to an aggressive environment, e.g., reactive ion etch and plasma etch. Due to the aggressive nature of the etch processes, the etch chamber materials must be selected carefully for reliable wafer processing. Therefore, the innermost etch chamber components are typically fabricated from quartz glass. Etching of pure quartz glass theoretically results in liberation of only silicon and oxygen species. These are less harmful to the wafer, as compared to transition metals and other elements, which would modify the composition and therefore the semiconducting properties of the wafer.
One example of a chamber component is a quartz glass window. In one configuration, the quartz glass window serves as a partition between the chamber atmosphere and the energy source. Because the window is typically positioned above the semiconductor wafer to be etched, it is imperative that the quartz glass window is as chemically pure as possible, i.e., having less than 50 ppm impurities. It is also imperative that the window has a very low concentration of bulk defects, e.g., foreign material inclusions and bubbles. Such bulk defects, when exposed to the etching atmosphere at the surface of the quartz glass window, can cause inhomogeneous etching of the window, thus generating quartz glass particles. Loose particulate matter within the etch chamber can be severely detrimental to the wafer. The size of such particles (1 to 10 microns) relative to the etched features (about 50 nanometers) on the wafer surface makes the particles potentially quite destructive. These particles may block gates and destroy conductive vias on the wafer or contaminate the wafer with impurity elements. Therefore, wafer etching chambers require quartz glass windows that etch slowly and uniformly without generating particulates.
Making quartz ingots using a sand-based flame fusion process is known in the art. Generally, this involves feeding a particulate quartz material through or near an oxy-fuel flame to gradually build up a massive glass ingot through an accretion process of relatively slow deposition rates, e.g., at 5 lbs/hr or less. The flame fusion process has the advantage of exposing individual particles to the full power of the heat source. However, this deposition-oriented process has the disadvantage of exposing each particle to contamination in the feed system and in the furnace atmosphere with each sand particle being an opportunity to form a defect in the ingot. The individual sand grains are exposed to the heat of the oxy-fuel flame and the product of the combustion reaction, water. The exposure yields quartz articles with hydroxyl concentration of greater than 150 ppm, which changes the temperature dependent viscosity of the fused glass, thus limiting its end-use applications.
Published application JP-61122131A discloses a glass ingot manufacturing device and glass ingots made thereof. U.S. Pat. No. 4,612,023 discloses a method for manufacturing stria-free, bubble-free and homogeneous quartz glass plates. U.S. Pat. No. 6,415,630 discloses an apparatus for producing a homogeneous quartz glass plate without streaks, in which the melting pot and the quartz glass rod undergo a relative motion perpendicular to the longitudinal axis of the rod.
In the prior art, quartz glass rods may be used as feedstock to form larger glass shapes. However, interfacial defects associated with material accumulation and overlay may still be experienced. In one prior art process, in which a laminate-type configuration to build up material (laying successive layers of softened glass on top of each other) is used, interfacial defects may result from a number of causes. These include entrained gas bubbles, entrained discrete impurity particles, entrained chemical impurities, and fold lines. Additional process steps may be required to reduce or eliminate such defects in the fused mass. Additionally, the process requires the use of refractory molds or containers to form the final shape of the glass ingot. Refractory in contact with the fused glass ingot is a source for contamination that can cause defects in the ingots. Furthermore, thermal expansion mismatch between the fused quartz glass and the refractory can cause spalling or cracking in the ingot.
There is a still need for ultra-low defect quartz articles from which such components for semiconductor processing assemblies can be fabricated, i.e., ingots with a hydroxyl concentration of less than 150 ppm and a total defect concentration (bubbles and inclusions greater than 10 micrometers in diameter) of less than 150 per cm3.